Down gauging of sheet thicknesses and/or new applications in severe corrosion environments for brazed aluminium heat exchangers requires more sophisticated materials to be used in these products with improved corrosion performance in the final brazed product. In common for these products is that the joints are created by a filler metal that has been applied on one or several sides of the different components that build up the unit prior to brazing. The most common configuration is to use a rolled strip with a non melting core and a cladding consisting of an Al—Si alloy. This cladding normally contains about 4-13% silicon that melts during brazing at about 600° C. The joints form by capillary flow of the filler metal to the desired joint sites and solidify to form a solid metallic connection between individual components.
Brazed heat exchangers consist of different parts like tubes, fins, plates etc. brazed together. Different alloys are traditionally used for tubes, plates etc. and these components are often protected from corrosion by electrochemically sacrificial fins. In some applications and heat exchanger positions this is not enough but the tubes/plates need a very good corrosion performance on their own. Different designs for brazed heat exchangers can use different solutions for tubes, fins etc. and the braze clad can be applied on either ones. Often the braze clad is applied on tube and plate materials.
The reason for the use of different alloys in different types of components is that in brazed exchangers it is normally necessary to chose different alloys in the different fin, tube, and plate components to avoid corrosion to perforation of tubes and plates by sacrificing the fins. The use of different alloys is often done by alloying the fins with Zn to reduce their corrosion potential to an adequate level compared to other parts of e.g. a brazed heat exchanger. In a consequence to this, materials used for tubes and plates normally have additions of Mn and Cu with the aim to increase their corrosion potential.
A further challenge today is to manufacture light-weight components for the automotive market. A lot of research is therefore directed to the ability to reduce the weight of heat exchangers by using thinner strip materials without sacrificing other product and manufacturing properties. To be able to do this it is necessary to create new materials with higher post braze strength compared to the materials conventionally used, but still with adequate corrosion properties. For tubes and plates this means that they should normally be sacrificially protected by other parts of the heat exchanger by having a higher corrosion potential than the other parts. Achieving a higher post-braze strength is quite complicated without jeopardising the corrosion resistance and the resistance to liquid core penetration during brazing. Liquid core penetration significantly reduces the corrosion resistance in brazed products. Only when these requirements are met consistently by the new materials, this will allow the use of thinner tubes with a high post brazed strength, thereby reducing the weight compared to the products used today.
So called Long Life alloys have been developed giving improved corrosion performance by creating a sacrificial surface layer after brazing by creating a lower corrosion potential in the surface than in the centre of the core. These alloys are primarily aimed for tube and plate applications where the corrosion property demand is very large. The lower corrosion potential on the surface is due to the existence of more manganese and copper in solid solution in the core centre than in the core surface. This is due to diffusion of especially silicon and copper between the core and the braze clad in combination with core composition and prior processing. A sacrificial surface can also be created by alloying a clad layer with zinc that diffuses towards the core centre during brazing. A higher zinc level at the surface after brazing will be sacrificial compared to the core centre. This principle is used for both water side corrosion protection by a non melting cladding and for melting braze claddings.
Generally, strip materials can be delivered in different tempers prior to forming. This can lead to formability problems if delivered in the deformed conditions of primarily H1X (e.g. H14 or H16) but to a lesser extent also H2X (e.g. H24 or H26). If delivered in O temper and deformed prior to brazing, problems can arise with so called liquid film migration (LFM) during brazing giving poor brazing performance and very poor post braze corrosion performance. Liquid core penetration during brazing will deteriorate the brazing performance and post braze performance, i e corrosion performance. These problems become more severe the thinner the final gauge used in the final product. Especially the very strong demand for very good post braze corrosion performance is difficult to meet.
Multi layer concepts are available where either the core must be protected from the braze clad during brazing and/or the post braze corrosion protection must be improved.
In a previous method known disclosed in U.S. Pat. No. 6,555,251 a four-layer material is produced where the interlayer has a higher content of Si than the core material. This material does however not possess sufficient corrosion resistance and resistance to braze penetration.
Another method for producing strip or sheet for heat exchangers is known from U.S. Pat. No. 6,743,396 in which an alloy is described containing ≤0.5% Fe, 1.0-1.8% Mn, 0.3-1.2% Si, ≤0.3% Mg, ≤0.1% Cu, ≤0.1% Zn, ≤0.1% Ti, 0.05-0.4% Cr+Zr, ≤0.15% Sn the remainder aluminium and unavoidable impurities, the ratio % Sn/% Si being ≥0.03. Ingots are cast, which are subsequently preheated to an initial rolling temperature less than 520° C. for at most 12 hours and hot rolled to a thickness between 2 and 10 mm with a final hot rolling temperature not less than 250° C. A final annealing is given at a temperature of at least 300° C., which means that the material is fully or substantially recrystallised. In this document nothing is said about core penetration during brazing and the resistance to corrosion after brazing is not mentioned. The high final annealing temperature would normally give a fully or partially recrystallised structure according to the patent description by the inventors. Nothing is said about the use of this material as an interlayer.
It has been shown in the practical testing of the material produced according to the previously known methods that the properties of the aluminium strip are insufficient for certain applications when the manufacturers need to down gauge. This particularly applies for the high post-braze strength combined with the high corrosion resistance and low susceptibility for liquid core penetration of the material.